1. Field of the Invention
The present invention relates to a method of printing an image formed of 2-dimensionally arranged pixels, and also to a printer such as an electronic photography apparatus or a thermal printer, which is designed to print an image formed of 2-dimensionally arranged pixels.
2. Description of the Related Art
A laser printer comprises a photosensitive member and a laser diode. The laser diode is driven in accordance with input image, thus emitting a laser beam to the photosensitive member. The surface of the photosensitive member is thereby scanned with the laser beam.
An item of the image data, which represents a pixel, is a signal S1 having the waveform shown in FIG. 1. As is evident from FIG. 1, the signal S1 is at either a high (H) level or a low (L) level for a period T.sub.1 during which the photosensitive member is scanned to form the pixel on its surface. The laser diode is driven by a drive signal S2, which has the same wave form as the image data signal S1 as is illustrated in FIG. 1. Hence, the laser diode emits a laser beam for the period T.sub.1, to form a pixel on the photosensitive member.
The electric resistance at the surface of the photosensitive member changes with the amount of light applied to the member. The electric charge of the surface of the member attenuates in accordance with the time the member is exposed to light. Hence, when the laser diode is driven by the signal S2 (FIG. 1) and emits a laser beam to the photosensitive member for the period T.sub.1, a circular pixel will be formed on the member, which circumscribes a square one-pixel region as is shown at (a) in FIG. 2.
When driven by two or more consecutive drive signals S2, the laser diode continuously emits a laser beam to the photosensitive member, thereby forming pixels which overlap one another as is shown at (b) in FIG. 2. As can be understood from FIG. 2, these overlapping pixels form a true black image since there are left no blank spaces among them. Therefore, the drive signals S2 are good for printing true black images.
The drive signals S2 shown in FIG. 1 are far form desirable for printing gray scale images by means of a Bayer-type dither. Nor are they desirable for printing characters, particularly complex ones (e.g., Chines characters), each consisting of many strokes. When driven by a signal S2, the laser diode emits a beam for the period T.sub.1, whereby a pixel will be formed which circumscribes the one-pixel region. Hence, even if some of consecutive drive signals S2 are at a low level as in the case of printing a gray image or characters, the resultant pixels overlap in most cases, as is illustrated at (c) in FIG. 2.
To print clear-cut gray images or characters, the laser diode can be driven by a signal S3 which is at the high level for a period T.sub.2 shorter than the period T.sub.1 as is evident from FIG. 1. When driven by the signal S3, the laser diode emits a laser beam to the photosensitive member for the period T.sub.2, forming a pixel on the member. This pixel is small, circumscribed by the square defining the one-pixel region as is shown at (a) in FIG. 3. Hence, if some of consecutive drive signals S3 are at a low level, the resultant pixels are discrete, forming a clear-cut gray image or distinct characters as is illustrated at (b) in FIG. 3. Obviously, the drive signals S3 are good for printing gray images and complex characters.
The signals S3, which remain at the high level for the period T.sub.2 (T.sub.2 &lt;T.sub.1) are not good for printing a true black image, however. When driven by two or more consecutive drive signals S3, the laser diode intermittently emits laser beams, each for the period T.sub.2, thereby forming discrete pixels, with blank spaces among them, as is illustrated at (c) in FIG. 3. The discrete pixels, thus formed, fail to define a true black image.
The above-described problem with the conventional laser printer is also inherent in other types of electrophotographic printers, such as a light-emitting diode (LED) printer, a liquid-crystal printer. Further, the problem is found in thermal printers such as a heat-transfer printer, ink-jet printers, and electrostatic printers.